Air temperatures near the Arctic surface have risen almost twice as much as the global average in recent decades – and melting sea ice is the main cause. This process will continue to accelerate, according to James Screen and Ian Simmonds of the University of Melbourne, who revealed their results in Thursday’s edition of Nature.
“The sea ice acts like a shiny lid on the Arctic Ocean,” Screen explains. “When it is heated, it reflects most of the incoming sunlight back into space. When the sea ice melts, more heat is absorbed by the water. The warmer water then heats the atmosphere above it. This feedback system has warmed the atmosphere at a faster rate than it would otherwise.” “It was previously thought that loss of sea ice could cause further warming,” adds Simmonds. “Now we have confirmation this is already happening.”
Using satellite observational data from the European Centre for Medium-Range Weather Forecasting, Screen and Simmonds found that the warming was concentrated in the lower atmosphere. Warming just above the planet’s surface from 1989 to 2008 was especially strong in autumn and winter, equating to temperature rises of 1.6°C per decade. With less ice cover, the upper ocean warms more in summer and releases its excess heat more effectively in winter.
Scientists previously believed that air transported from warmer areas toward the North pole, or changes in cloud cover, might have caused higher Arctic temperatures. Screen and Simmonds’ study showed that cloud cover had little impact, while the effect of changes in ice cover were especially strong. “The emergence of strong ice–temperature positive feedbacks increases the likelihood of future rapid Arctic warming and sea ice decline,” they write.
The consequences of melting sea ice for air temperature are currently more significant than its contribution to sea level rise, if research published this week in Geophysical Research Letters is anything to go by. Yet the sea level study performed by British scientists at the University of Leeds and University College London also highlights that those temperature rises could in turn quicken ice melting still further.
James Shepherd and his colleagues have published the first ever estimates of how the mass of Earth’s floating ice is changing overall. They find that altogether between 742 cubic kilometres – or roughly the volume of Lake Titicaca in Peru, which is the 14th largest lake in the world – of floating ice was lost on average each year between 1994 and 2004. However, this contributes just 0.05 mm/year to the current 3.1 mm/year rate of sea level rise. The remainder or the rise is largely caused by increased temperatures making the water in the oceans expand.
“Overall, increases in sea level due to the loss of Arctic sea ice and the collapse of several Antarctic ice shelves have been mitigated by gains in Antarctic sea ice and an overall thickening of the remaining Antarctic ice shelves,” Shepherd and co-workers write. Because floating ice is sensitive to small changes in temperatures, the contribution of melting ice to sea level rise might increase as global warming continues. “A 0.1°C rise in ocean temperature beneath Antarctic ice shelves alone would lead to an estimated 10 mm rise in global sea level,” the team writes.
Meanwhile, three days of climate negotiations between 45 states will begin tomorrow in Petersberg, Germany, in advance of a more extensive conference in the same country later on in the month. The German government says that the interim talks “will focus on the question of how progress can be made on important negotiating issues and what role the Copenhagen Accord can play in achieving this.” After Joeri Rogelj and his colleagues pointed out how unsatisfactory the outcome of Copenhagen was on Simple Climate this week, the need for successful talks is greater than ever.